Your Phone Can Tell If an Earthquake Is About to Hit. Here's How. Millions got an early notice about the 6.4 magnitude earthquake that rocked Northern California.

More than 3 million Northern Californians got an early earthquake warning, which gave them precious seconds to duck and cover before it hit on Tuesday. At least two people were killed in the 6.4 magnitude quake that left thousands without power and damaged homes and roads.

The early warning was delivered via the California Governor's Office of Emergency Services' MyShake app, as well as alerts preprogrammed into the phones of Google and Android users.

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Robert de Groot, from the U.S. Geological Survey's ShakeAlert operations team, explained to the Los Angeles Times how the alert system works. In brief, the ShakeAlert team uses seismometers that pick up the earthquake as it nears the surface. That data goes straight to a processing center which converts it into alerts that are sent out to users.

The Tuesday morning alerts gave users up to 20 seconds to prepare, which might not sound like a lot, but it is enough to take cover, and also to mentally prepare for what is about to happen.

Twitter user @MtShasta wrote in part: "Thank goodness for shake alert! I'd have fainted otherwise! 8+ second warning of severe shaking! Wow!"

Thank goodness for shake alert! I'd have fainted otherwise! 8+ second warning of severe shaking! Wow! I'm in Mt Shasta! It was jerking up here! ?

— NorthwestNomad (@MtShasta) December 20, 2022

Groot told the Times that the team was thrilled by this first test of the system. "I believe it may be the actual biggest event that we've had so far," he said. "The system is doing exactly what it should be. This is actually a really big success for us."

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His sentiment was echoed by Mark Ghilarducci, director of the California Governor's Office of Emergency Services, who told the outlet: "The system did operate as we had hoped, and [as] we've been working to design."

If you live in an area prone to earthquakes, perhaps an alert app would be a better use of your phone storage than Fruit Ninja.